This invention relates to a gas turbine airfoil with internal serpentine passages for cooling purposes.
Turbine airfoils are subjected to the very high temperatures of the hot gas driving the turbine. In order to prevent damage to the airfoils due to the high temperatures and assure a reasonable lifetime the airfoils are cooled externally and internally by a cooling medium, typically cooling air bled from the compressor of the gas turbine. Internal cooling of the airfoil is realized by several passages within the airfoil between the pressure sidewall and the suction sidewall of the airfoil. The passages typically extend spanwise from the root of the airfoil to its tip. Some of the passages consist of a single passage with an exit port near the tip of the airfoil and/or several film cooling holes on the edge or on the side wall of the airfoil. Other passages follow a serpentine path allowing the cooling air to flow for example from the root to the tip and around a 180xc2x0 turn. From the tip it extends towards the root and around a further 180xc2x0 turn that directs it again toward the tip where it finally exits through exit ports or film cooling holes. Serpentine cooling passages of this type are disclosed for example in U.S. Pat. No. 5,403,159. They allow for a high internal heat transfer with a minimum amount of cooling air.
FIG. 1 shows a radial cross-section of a typical airfoil 1 of the state of the art with several internal passages extending radially inward and outward between a root section 2 and a tip 3. A first internal passage 4 extends from an entry opening 5 in the root section 2 radially outward to the tip 3 of the airfoil. Cooling air can flow from the root section 2 through the passage and exit via several cooling slots 6 along the trailing edge 7 as well as through a tip hole 8. A second internal passage 10 extends from an entry opening 11 radially outward along the leading edge 12 of the airfoil. Cooling air flows through this passage 10 and exits via a tip hole 13 and through several rows of film cooling holes 31 drilled through the leading edge 12 of the airfoil. A serpentine passage comprises an entry opening 14 at the radially inner end of the root section, a first passage 15a extending radially outward with a tip hole 17. At the tip a 180xc2x0 turn 16 leads to a passage 15b extending radially inward. At the radially inner end of the passage a second 180xc2x0 turn 18 leads to a third passage 15c extending radially outward to a tip hole 19. Cooling air flowing through the straight and serpentine passages cool the airfoil from within by impingement cooling and exits through the film cooling holes on the edges of the airfoil 1 and/or through the tip holes. Other typical airfoils have several serpentine cooling passages or serpentine passages comprising five passages with four turns.
Airfoils with internal serpentine geometry for the cooling passages are typically manufactured by an investment casting process, which utilizes a ceramic core to define the individual internal passages. Following the casting the ceramic core is removed from the airfoil by a leaching process. The film cooling holes on the edges and sidewalls of the airfoil are then realized by a laser drilling process. This process involves, previous to the actual drilling, the insertion of a backing or blocking material which limits the laser radiation to the desired locations of the film cooling holes and prevents damage to the passage walls and other inner surfaces of the airfoil. Such a method is disclosed for example in U.S. Pat. No. 5,773,790. It uses a wax material as a blocking material.
During the process of casting the internal passages it is often difficult to maintain the separation of the passages in the cores due to thermal strains caused by differential heating and cooling rates of the core and surrounding metal.
A current practice to maintain the separation of the serpentine passages 15a,b,c and to support the core during the casting process utilizes conically shaped features in the core. These conical features are formed as part of the core and extend from the root section through an opening in the wall of the 180xc2x0 turn 18 and into the passages 15b and 15c. After the part is cast and the core is leached out, the conical feature is closed off with a spherically shaped plug 30 that is brazed into place.
The conical feature maintains a near constant cross-sectional area and outer radius of the outer wall through the 180xc2x0 turn in order to minimize pressure loss. Typical measured pressure losses through the turns are usually  greater than 1.5 times the dynamic pressure of the cooling air stream entering the turn.
However, the conical feature presents a weak spot in the core where it can break resulting in movement of the passages 15b and 15c, the turn 18, and the root section of the passage 15a. 
Following the casting process and the leaching out of the core material, a backing material must be inserted into the cooling passages for the laser drilling of film cooling holes.
As the passages 15b and 15c following the 180xc2x0 turns are not easily accessible from either end, it is difficult to fill these passages with backing material. In current practice this problem is circumvented with the use of a liquid wax, which is typically hot injected into the opening 14 until wax is seen exiting from the tip hole 19. After the completion of the laser drilling, the waxen backing material is removed from the airfoil by heating the airfoil and burning the wax. This practice has shown however, that the use of wax as a backing material does not sufficiently absorb the laser energy, and therefore provides only limited protection from so-called back wall strike.
This invention provides an airfoil and a method of manufacturing the airfoil that comprises internal cooling fluid passages arranged in a serpentine path having one or more radially outward extending passages and one or more radially inward extending passages, which are connected by turns of approximately 180xc2x0. In particular, the invention provides an airfoil and a method of manufacturing such an airfoil that enables improved maintenance of the passage separation during casting and the use of a blocking material for a laser drilling process that provides greater shielding compared to the blocking material used in current manufacturing methods.
An airfoil comprises internal cooling air passages arranged in a serpentine manner having one or more radially outward extending passages and one or more radially inward extending passages, and turns of approximately 180xc2x0 providing fluid connection between a radially inward and a radially outward extending passage. A radially inward extending passage in a serpentine passage is defined by the inner surfaces of the pressure and suction sidewalls of the airfoil. A first wall and a second wall separate the radially inward extending passage from neighboring passages. A radially outward extending passage in a serpentine passage following the radially inward extending passage in the fluid flow direction is defined by the inner surfaces of the pressure and suction sidewalls of the airfoil, the second wall, and a third wall. The third wall can be a separating wall to a further cooling passage or the leading or trailing edge wall of the airfoil.
According to the invention, each radially inward extending passage is in fluid connection with the next radially outward extending passage in the direction of the cooling fluid flow by means of a root turn. This root turn is defined by the first wall of the radially inward extending passage and the third wall of the radially outward extending passage, which both extend to the radially inner end of the root section of the airfoil. The root turn is further defined by a member that closes off the root turn at the radial inner end of the root section of the airfoil and is attached to the radially inner ends of the first and third walls of the passages.
The radially inward extending passage and the radially outward extending passage are thus combined below the airfoil platform. There is therefore no need for a curved outer wall for a turn at the level of the airfoil platform. This geometry for an airfoil avoids a 180xc2x0 turn with a curved side wall and avoids the necessity of a hole for insertion of the conical features and the subsequent closing of the hole with a ball braze.
In a method for manufacturing the airfoil according to the invention, a ceramic core is used for the casting of the internal passages between the pressure and suction sidewalls of the airfoil. The ceramic core forms the root turn at the radially inner end of the root section of the airfoil in the casting process. Following the casting process, the ceramic material making up the core is leached out of the cast airfoil. Thereafter, strips of a thermoplastic resin sold under the trademark Teflon(trademark) m are inserted into the passages, and film cooling holes are drilled. During the drilling process the Teflon(trademark) strips protect the surrounding cast material from the laser radiation. After the drilling process the Teflon(trademark) strips are removed and a member is placed at the end of the root turn in order to close it off.
During the casting process the root turn is open-ended, allowing good access for the application of additional core supports in the region of the turn. This provides improved control over the wall separation and the location of the turn. Following the casting process and removal of the core, the open turn enables excellent access for the insertion of a backing material necessary to perform the laser drilling of film cooling holes.
This backing material is not necessarily fluid such as a waxen material, but instead can be made of a stiffer material such as strips of Teflon(trademark) (PTFE). Teflon(trademark) provides an improved shielding compared to the waxen material used in current practices. Following the laser drilling process the Teflon(trademark) strips are again easily removed, after which each root turn is closed off with a member such as an end plate welded or brazed to the radially inner ends of the sidewalls of the serpentine passages combined by the root turn.
Furthermore, the open root turns enable an improved leaching out of the core material after the casting process.